The smoothest bore is the wrong bore
Buyers ask for a mirror finish because a mirror looks precise. A piston seal disagrees. The seal needs a controlled amount of oil held in the surface texture at the moment the piston reverses, and a polished bore has nowhere to keep it. The first stroke after a dry start then runs boundary friction, the seal lip heats, and the wear rate climbs.
Honing solves this by cutting a deliberate pattern of crossing grooves. Those grooves are oil reservoirs. The flat areas between them, the plateaus, carry the load. Getting both right is the entire point of plateau honing.
The parameters, and what each one controls
| Parameter | Measures | Typical for hydraulic bores |
|---|---|---|
| Ra | Arithmetic average roughness | 0.2 – 0.4 µm |
| Rz | Average peak-to-valley height over sampling length | 1 – 4 µm |
| Rpk | Reduced peak height, the part that wears off in run-in | ≤ 0.3 µm |
| Rvk | Reduced valley depth, the oil retention capacity | ≥ 0.8 µm |
Ra alone is a weak specification. Two bores can measure Ra 0.3 µm while one has sharp peaks and shallow valleys and the other has flat plateaus and deep grooves. The first wears the seal, the second feeds it. This is why a serious cylinder drawing specifies Ra together with Rpk and Rvk, or names the honing process and the plateau requirement.
Cross-hatch angle
The angle is the included angle between the two sets of helical grooves left by the hone as it rotates and reciprocates. It is set by the ratio of spindle speed to stroke speed. Raise the rotational speed relative to the stroke and the angle flattens; raise the stroke speed and it steepens.
| Application | Typical cross-hatch angle | Why that range |
|---|---|---|
| Hydraulic cylinder tubes | 30° – 60° | Oil retention balanced against seal friction |
| Automotive engine bores | 40° – 60° | Ring seating and oil control |
| High-performance engines | 35° – 45° | Reduced oil consumption |
| Pneumatic cylinders | 20° – 30° | Minimal lubrication available |
Within the hydraulic range the trade is directional. A steeper angle around 60° holds more oil and generates more seal friction. A shallower angle near 30° lowers friction and holds less. Most hydraulic work converges between 40° and 50°.
Plateau honing, in one paragraph
Conventional honing leaves a surface with sharp peaks standing above the grooves. Those peaks get shaved off during the first hours of service, and the debris goes into the oil. Plateau honing adds a final light pass with fine abrasive that removes the peaks while leaving the valleys intact. The result is low Rpk with retained Rvk, which means short run-in, low initial wear debris, and oil where it is needed from the first stroke.
How to check a tube without a laboratory
Three field checks catch the majority of bad bores before they reach assembly.
- Lighted bore inspection. Shine a bright light down the axis from one end and look from the other. A good bore shows even diffuse reflection. Alternating light and dark bands spiralling along the length indicate chatter. Straight bright lines are scores; intermittent dark lines can be laps or folds in the parent material.
- Gloved tactile check. With a clean lint-free glove, run fingertips along the bore. Sensation should be uniform. Ridges, steps or texture changes that you can feel are failures regardless of what the roughness gauge says.
- Roughness at three stations. Measure the same tube at both ends and the middle. A supplier who polishes the ends to pass inspection while the middle runs coarse is a known failure pattern, and single-point measurement hides it.
What honing can and cannot fix
Honing corrects form errors in the bore it follows: roundness, taper, cylindricity, straightness. It cannot correct positional errors. If the bore axis is offset from an external datum or angled relative to a mounting face, no amount of honing moves it. Those errors belong to the drilling, boring or fixture setup upstream, and a drawing that calls for tight honing tolerances without controlling position will produce scrap at assembly rather than at the honing machine.
Specifying finish on an enquiry
State four items: target Ra, target Rpk and Rvk if the seal package is sensitive, cross-hatch angle range, and whether a plateau finish is required. Also state the seal material. A polyurethane seal tolerates a different finish from a filled PTFE one, and the seal supplier’s own recommendation should outrank a generic catalogue figure.
Our honed bore finish is controlled to [ORISE: confirm routine Ra range and cross-hatch angle control value]. Where a value has not been measured and recorded on our own line we mark it not published rather than quoting an industry average as our own. Process detail sits on the deep hole boring and honing page, and the measuring instruments we use are listed under inspection equipment. For a specific requirement, send the drawing through the RFQ page.